The role of water in APCI-MS online monitoring of gaseous n-alkanes

In atmospheric pressure chemical ionization mass spectrometry (APCI-MS), [M−3H+H2O]+ ions can deliver analyte-specific signals that enable direct analysis of volatile n-alkane mixtures. The underlying ionization mechanisms have been the subject of open debate, and in particular the role of water is insufficiently clarified to allow for reliable process analytics when the humidity level changes over time. This can be a problem, particularly in online monitoring, where analyte accumulation in the ion source can also occur. Here, we investigated the role of water during APCI-MS of volatile n-alkanes by changing the carrier gas for sample injection from a dry to a wetted state as well as by using 18O-labeled water. This allowed for a distinction between gaseous and surface-adsorbed water molecules. While adsorbed water seems to be responsible for the desired [M−3H+H2O]+ signals through surface reactions with the analyte molecules, gaseous water was found to promote the formation of CnH2n+1O+ of different (and analyte-independent) hydrocarbons, revealing a reaction with hydrocarbon species which accumulated in the ion source during continuous operation. At the same time, gaseous water competed with analyte molecules for ionization and thus suppressed the formation of alkyl (CnH2n+1+) and alkenyl (CnH2n−1+) ions. The results reveal a memory effect due to hydrocarbon adsorption, which may cause severe interpretation difficulties when the ionization chamber undergoes sudden humidity changes. The use of [M−3H+H2O]+ for n-alkane analysis in alkane/water mixtures can be facilitated by constantly maintaining high humidity and hence stabilizing the ionization conditions. Graphical abstract Supplementary Information The online version contains supplementary material available at 10.1007/s00216-024-05431-5.

The overlap of identical [M−H] + fragment ions for different n-alkanes complicates an analysis of mixtures [8].In our previous work, we showed that oxygen-containing [M−3H+H 2 O] + ions can be beneficial for quantifying volatile and semi-volatile (~C 5 -C 20 ) n-alkanes in gaseous streams, since they produce analyte-specific signals [16].Hardly any fragments with the molecular formula C n H 2n+1 O + were observed, which allowed for an easy and reproducible [M−3H+H 2 O] + signal allocation, even in mixtures of multiple n-alkanes.Still, it remained an open question how [M−3H+H 2 O] + were formed and why no overlapping fragment ions were observed.Moreover, especially for larger alkanes, long-tailing ion signals were detected, indicating an analyte accumulation in the ion source.In a time-optimized online sample injection application without cleaning procedures in between, this continuous analyte accumulation in the ion source may lead to an interference of the related signals with actual sample signals.
The formulation [M−3H+H 2 O] + assumes an ion formation with water, and several researchers agree on a hydration mechanism [3,6,9,14], e.g., by but an oxidized [M−H+O] + molecule is also imaginable.Nyadong et al. confirmed water as oxygen source by adding 18 O-labeled water to the system [6].However, other researchers ruled out water as oxygen source and determined a reaction with ozone (O 3 ), produced by O 2 during corona discharge, as the mechanism responsible for the formation of [M−H+O] + [17].Evidently, APCI ionization routes of saturated hydrocarbons are still under debate and may even differ, depending on applied ionization conditions.
Water is generally known to be omnipresent during APCI-MS, since air humidity is usually inevitable.But in most literature studies, the role of water is limited to the source of H 3 O + ions and water clusters [18][19][20], which may act as proton donors in subsequent ionization steps (as, e.g., in Eq. 3).Oxygen-containing ions, such as [M−3H+H 2 O] + / [M−H+O] + , are typically mentioned as a side issue only.However, a clear understanding of the processes leading to their formation is crucial when these ions are to be used for (5) hydrocarbon quantification, especially in applications where water is produced along with hydrocarbons (e.g., biomass pyrolysis [21] or Fischer-Tropsch synthesis [22]).
In this study, we investigate the role of water during online APCI(+)-MS of gaseous n-alkanes.In this context, we particularly differentiate between two water states, i.e., gaseous water and water adsorbed to surfaces within the ion source.The results lead to the proposition of an ionization scheme for the formation of [M−H] + , [M−3H] + ions, their fragments and of [M−3H+H 2 O] + ions.Moreover, the problem of hydrocarbon accumulation in the ion source and the accompanying influence of water is outlined.We discuss the meaning of the evolved APCI mechanism for an online analysis application of n-alkane/water mixtures and provide a first approach for system stabilization.

Experimental setup
Gaseous samples of diluted n-alkanes were analyzed using a Q Exactive Plus Orbitrap mass spectrometer (Thermo Fisher, Waltham, MA, USA) and an 8860 gas chromatograph (Agilent, Santa Clara, CA, USA) for reference.The hydrocarbons were injected into an evaporation chamber via a syringe pump and transported by diluted syngas mixture (CO/H 2 /N 2 = 1:2:1) to both instruments.Argon was used as carrier gas to flush the sample into the APCI ion source.In order to add water to the ionization chamber, a water-filled tube (~ 5 ml capacity) was inserted into the argon carrier gas pipeline, so that the gas stream could be enriched with water before it flushed the sample into the MS.A three-way valve allowed for switching between dry argon carrier gas and water-enriched argon carrier gas (Fig. 1a).The MS sample injection was performed automatically between 6 s ≤ t meas < 15 s, and the overall measurement duration was about 5 min (289.8s).An illustrative plot of signal intensity over time during one sample measurement is shown in Fig. 1b.A signal integration over time results in a peak area, which is used for a correlation with the analyte concentration.More details about the experimental setup, the APCI-MS method, signal reproducibility as well as about the reference GC measurements, can be found in our previous study [16].

Effect of gaseous water
A C 5 /C 6 /C 7 /C 10 /C 12 /C 14 (mole fractions y i = 0.403/0.282/0.198/0.068/0.033/0.016)n-alkane mixture was injected by a syringe pump into the evaporation zone with a constant flow rate of 6 µl/min.The hydrocarbons were transported by a diluted syngas mixture (CO/H 2 /N 2 = 1:2:1, 200 ml STP /min) to both the MS instrument and a reference gas chromatograph.The MS argon carrier gas flow was started in dry argon mode.After reaching a steady state, the argon was switched to the water bath filled with 18 O-water.This way, the effect of gaseous water molecules could be distinguished from water molecules that were present in dry argon mode.

O injection
Before performing another alkane experiment, a H 2

18
O sample of 600 µl was directly injected into the ionization chamber for 3 min.This aimed to study the effect of incoming water on possible hydrocarbon accumulation and further to enrich the ionization chamber with H 2 18 O molecules, leading to partial H 2 18 O condensation and surface accumulation.To enhance this process, the vaporizer temperature T vap was lowered to 100 °C.Mass spectra were continuously measured during the injection and the time afterwards to monitor possible effects of the strong humidity increase.

Effect of adsorbed water
After the direct H 2 18 O injection, the ion source temperature was brought back to normal operation conditions for a C 10 injection experiment.n-decane was injected with a constant flow rate of 1 µl/min into a N 2 gas stream (50 ml STP /min), evaporated and guided to the MS device.It had been found that, using the previous MS sheath gas conditions from the sections "Effect of gaseous water" and "Direct H 2 18 O injection" (sheath gas: 2, sweep gas: 10), ionization efficiency was rather low, so that small amounts of 18 O-species were hardly detectable.Thus, in this experiment, the MS sheath gas flow was increased to 40 and the sweep gas was turned off, which enhanced signal intensities and decreased the lower detection limit.Similar to the other experiment, the argon carrier gas was dry at first, then switched to the water bath filled with 18 O-water and, after reaching a new steady state, switched back to dry argon mode.

Effect of gaseous water
Two exemplary mass spectra of the C 5 /C 6 /C 7 /C 10 /C 12 /C 14 mixture, showing one MS scan during a measurement with dry argon carrier gas flow and one MS scan during a measurement with H 2 18 O-wetted argon flow, are presented in Fig. 2a and b, respectively.
Both scans correspond to an equal point in time during their measurement (t meas = 10.8 s).The spectra look qualitatively similar: Most abundant species are alkyl ions (C n H 2n+1 + ), especially with chain lengths n = 4, 5 and 6 (m/z 57.0699, m/z 71.0855, and m/z 85.1012, respectively).This is in agreement with our previous study, and highlights the difficulty in quantifying alkane mixtures using [M−H] + , since the degree of fragmentation is high and hence the attribution of peaks to different analytes is infeasible.Here, for instance, it is impossible to distinguish between m/z 71.0855 (C 5 H 11 + ) originating from n-pentane or from a fragmentation of one of the higher n-alkanes.Quantitatively, the additional presence of water molecules leads to a significant decrease in signal intensity (see Fig. 2b).This decrease was observed regardless of an argon enrichment with H 2 18 O or conventional H 2 16 O (not shown here).Evidently, the addition of gaseous water inhibits the formation of [M−H] + ions and their fragments.Ions, which would suggest a reaction of alkyl ions with water to other species, were not observed.Hence, it seems that water molecules competed with analyte molecules and were preferably ionized.Main ionization products were presumably H 3 O + and water clusters with m/z < 50, which could not be detected.This indicates that under the applied ionization conditions, [M−H] + and fragment formation is not due to proton transfer reactions (Eqs.3 and 4), since a higher availability of H 3 O + ions would suggest a signal increase rather than a decrease.Instead, another mechanism, e.g., hydride abstraction, could be predominant.This is contradictory to the results of Manheim et al., who ruled out hydride abstraction and determined a proton transfer mechanism to be dominant [15].Still, their mechanistic study was performed under vacuum and lower temperature conditions, which could explain different reaction regimes.
With respect to oxygen-containing C n H 2n+1 O + ions, the influence of gaseous water is more complex.Figure 3  O-enriched argon carrier gas, respectively The role of water in APCI-MS online monitoring of gaseous n-alkanes ions with dry argon, which is reasonable, since H 2 18 O had not been added yet and the 16 O/ 18 O ratio in natural water is only about 0.2% [23].When the argon carrier gas was enriched with H 2 18 O, both isotopic ion groups increased in signal abundance, but in a different manner: C n H 2n+1 16 O + peak areas increased, most significantly for the longer analytes (~5% increase for C 5 H 11 16 O + vs. ~60% increase for C 14 H 29 16 O + ).In contrast, C 14 H 29 18 O + still showed no relevant signal, while ions with a shorter chain length, such as C 5 H 11 18 O + , were detected with high abundance (magnitude of 10 6 ).In addition, it is striking that the signal development of C 5 H 11 18 O + did not show a typical peak shape.Instead, the ions were continuously detected over the entire measurement period, i.e., even before the current sample injection.Hence, the added gaseous water interacted with hydrocarbon molecules, but obviously not with the ones of the current sample.The constantly detected C 5 H 11 18 O + signal must originate at least partly from accumulated hydrocarbons (from previous injections), and the "new" sample molecules (such as C 14 ) seem to be unaffected.This leads to two conclusions: First, hydrocarbon accumulation is present and becomes apparent when adding gaseous water.Incoming water seems to react with accumulated hydrocarbons, which are then remobilized and detected.This issue will be further discussed in the section "Direct H interaction with a different oxygen source.Two possible oxygen-containing reactants that are promoted by the presence of gaseous water are imaginable: (1) ions which are based on atmospheric O 2 and N 2 and which form hydrates with gaseous water, e.g., O 2 [1, 24]; (2) adsorbed water, which is predominantly H 2 16 O(ads), and increases in reactivity due to higher water partial pressure in the ion source (similar to heterogeneously catalyzed reaction rates).Although hydrates such as O 2 +• H 2 O n or NO +• H 2 O n might be formed during the ionization pro- cess, they were not detected here (n = 1 could not be detected anyway due to m/z < 50), and moreover, a transfer of only one oxygen atom from those ions to form [M−H+O] + is unrealistic.Hydrocarbon adducts with O 2 + or NO + seem more reasonable.Thus, the second route, i.e., an increased probability of primary hydrocarbon ion reactions with adsorbed water molecules forming [M−3H+H 2 O] + due to higher water partial pressure, is more plausible, especially since this also explains a prolonged signal detection due to slow wall interactions.However, this scenario still needs to be confirmed.

O injection
The observed reaction of water with accumulated hydrocarbons was further examined by a pure and direct H 2 18 O injection with no additional hydrocarbon analytes involved. of n = 3-10 were measured.Hence, there has been a significant accumulation of hydrocarbon molecules which were remobilized by the added water.This reinforces the observation of the previous experiment that gaseous water (presumably via H 3 O + ) reacts with adsorbed hydrocarbon molecules that did not originate from a current sample injection, but from former sample injections (Eq.9).Moreover, since for instance n-propane (C 3 ), n-butane (C 4 ), and n-octane (C 8 ) were never used as analytes before, a high fraction of adsorbed species evidently consists of adsorbed (and presumably neutral) fragments.
Remarkably, this ion formation was visible not only during the 3 min of H 2 18 O injection, but for several hours afterwards as well.Interestingly, smaller molecules were detected for a longer period than larger ones, since C 3 H 7 18 O + in particular was found to be a very persistent signal.This might be due to a higher relative polarity of short-chained fragments, which causes stronger interactions with the hydrophilic walls.
The inset in Fig. 3 shows an exemplary mass spectrum captured during this process.The C n H 2n+1 18 O + signals were the most abundant, highlighting that this type of oxygencontaining species is a particularly stable reaction product.The alkenyl cations C 5 H 9 + (m/z 69.0699) and C 6 H 11 + (m/z 83.0855) were also very abundant, which suggests that they were formed via a water-based pathway as well, but with subsequent water elimination (Eq.10).We propose the following ionization mechanism for gaseous H 3 O + ions with adsorbed neutral, saturated hydrocarbon molecules: When using APCI-MS as a continuous online monitoring method, this means of course a problematic situation, since signals from analytes and accumulated species overlap, and no reliable quantification can be made.However, we did not observe this phenomenon during our last study, where only n-alkanes (no water) were investigated.Hence, this problem seems to be significant only when water is added.This is an interesting mechanistic observation, as it means that nonpolar accumulated hydrocarbons may be effectively cleaned by polar water instead of nonpolar solvents due to remobilizing surface reactions.

Effect of adsorbed water
After the direct injection of H 2 18 O, we expected that the surface inside the ionization chamber would be at least partly covered with 18 O-labeled water molecules.This would allow us to check whether they interact with the analyte n-decane. (9) Furthermore, the effect of additional gaseous H 2 18 O could be monitored by switching between dry and wetted argon mode.Figure 5a and b  O + , as well as possible larger accumulated molecules which might be released by gaseous water.The inhibiting effect of water on alkyl and alkenyl cations during this experiment is shown in Fig. S3 in the Supporting Information.
Up to a time of ~1070 min, dry argon was used as carrier gas (white background).Both [M−3H+H 2 16 O] + and [M−3H+H 2 18 O] + (i.e., C n H 2n+1 O + with n = 10) were detected, while C n H 2n+1 16 O + and C n H 2n+1 18 O + fragments were hardly visible.Hence, this time, the chemical behavior of both C n H 2n+1 O + isotopes seems to be identical.Moreover, both [M−3H+H 2 16 O] + and [M−3H+H 2 18 O] + signals were detected with similar temporal behavior (insets in Fig. 5a and b at t ≈ 500 min).This indicates that the partial coverage of ion source surfaces with H 2 18 O was successful, and the formation pathway of [M−3H+H 2 O] + via adsorbed water was a valid assumption.No gaseous water was added up to this point, i.e., these ions probably originated from water molecules which were adsorbed to the ion source walls.It is still striking that, despite the previous H 2 18 O injection, the signal of [M−3H+H 2 16 O] + is about two orders of magnitude higher than [M−3H+H 2 18 O] + .The peak area fraction of [M−3H+H 2

18
O] + with respect to all [M−3H+H 2 O] + signals was about 0.51% during the first dry argon period.Apparently, 16 O-water was still predominantly available on the surfaces.It is likely that greater effort to exclude atmospheric humidity would be needed to replace H 2 16 O(ads) by H 2  18 O(ads) to a higher degree.Metal surfaces are known be covered with water [25,26], and the continuous exposure to atmospheric conditions still allows H 2 16 O to preferably occupy open surface sites.
The question arises as to which ions react with surfaceadsorbed water to form [M−3H+H 2 O] + .Different scenarios are imaginable, but since no fragments of C n H 2n+1 O + were observed, a reaction of early-formed (and thus not yet fragmented) analyte ions seems probable.If [M−H] + or [M−3H] + would react with adsorbed water, we would expect their fragments to behave analogously, and the C n H 2n+1 O + fragmentation pattern would look similar to that of the other ion types.Instead, we suggest a reaction mechanism based primarily on formed molecular radical cations: The role of water in APCI-MS online monitoring of gaseous n-alkanes This cannot be verified, and both intermediate reaction steps and more involved species are imaginable.Still, this reaction could explain the low degree of fragmentation of C n H 2n+1 O + ions: M +• ions are formed in a first step and hold a large amount of internal energy.By colliding with the ion source walls and reacting with adsorbed water, a part of that internal energy is transferred to the surface.Desorbing products possess significantly less internal energy and reach the detector without major fragmentation.When the argon stream was guided through the 18 O-water bath (t ~1070 min), both water-based mechanisms became visible: (1) The reaction of analyte ions with surface-adsorbed water (Eq.12) became more probable due to higher water partial pressure.Because the surface was covered with both H 2 16 O and H 2 18 O, the abundance of both [M−3H+H 2 16 O] + and [M−3H+H 2 18 O] + strongly increased.(2) Incoming gaseous water reacted with adsorbed hydrocarbons (Eq.9).Various C n H 2n+1 18 O + species were immediately detected after adding H 2 18 O, including chain lengths of n > 10, which cannot be fragments of n-decane but must originate from previous experiments.With continuous 18 O-water addition, their signal abundance again decreased.Thus, a part of the analyte signal [M−3H+H 2 18 O] + presumably originates from previous samples and not from ionized analytes, since the signal first increased strongly and declined subsequently.
The instantaneous release of adsorbed hydrocarbons is further demonstrated in the inset signal curve of C 12 H 25 18O + (blue) in Fig. 5b.It represents the moment where argon was switched from dry to 18 O-wetted mode (at about half time of the measurement).The C 12 signal increased immediately after that switching event, although sample injection already occurred minutes ago.
When the argon stream was brought back to dry conditions (t ~1750 min), [M−3H+H 2 O] + signals decreased again, showing the effect of decreasing water partial pressure.A memory effect was observed, as peak areas decreased rather slowly compared to the steep increase after the first switching event.This further supports the scenario of a surface-related reaction scheme, in which a water-saturated state should decline rather slowly.Hence, adsorbed water molecules would remain available for a long period.One could expect this behavior especially for [M−3H+H 2 18 O] + , since H 2 18 O was added for several hours.However, the signal of [M−3H+H 2 18 O] + decreased slowly but constantly (even below the level of the first dry argon mode), while the peak areas of [M−3H+H 2 16 O] + reached a steady state with a slightly higher abundance compared to the first dry argon mode.This could further emphasize that replacing H 2 16 O(ads) by H 2 18 O(ads) was rather unlikely, especially when only small amounts of H 2 18 O where added via argon.The available amount of adsorbed H 2 18 O molecules appears to originate primarily from the previous direct (liquid) H 2 18 O injection and hence decreased constantly afterwards.In contrast, H 2 16 O(ads) was still excessively available.

Proposal of an ionization scheme
Based on all experimental results as well as on previously described APCI ionization mechanisms, we propose a general scheme for the formation of M +• , [M−H] + , [M−3H] + , [M−3H+H 2 O] + ions and hydrocarbon fragments according to Fig. 6.The formation of [M−H] + ions and other alkyl ions decreased upon addition of water, although presumably more H 3 O + ions were available as proton donators.Hence, a predominant reaction route via proton transfer, as postulated by Manheim et al. [15], is questionable here.Instead, under the applied conditions, [M−H] + ions might be formed by hydride abstraction (Eqs. 1 and 2).Since this was not investigated further, both pathways are added to the scheme.[M−3H] + is formed by a subsequent H 2 elimination (Eq.5).Both ion types undergo significant chain fragmentation (Eqs.6 and 7).[M−3H+H 2 O] + ions (highlighted in blue) are suggested to be formed by a reaction of molecular radical ions with adsorbed water (Eq.12) and, due to an energy transfer to the wall, they do not suffer from major chain cleavage.Subsequent water elimination is possible, which enables an alternative route for [M−3H] + formation.Any neutral hydrocarbon species (analyte or fragment) can adsorb to the surface and is thus available to participate in the ionization process of subsequent samples.This accumulation or memory effect becomes visible when gaseous water is added, forming H 3 O + ions which remobilize adsorbed hydrocarbons and react to C n H 2n+1 O + ions (Eq.7).
These mechanistic considerations cannot be entirely proven, but they provide a suitable model for how water is involved during this APCI-MS study of n-alkanes.With respect to an online MS application, two important conclusions can be drawn from these results.First, regardless of whether [M−H] + , [M−3H+H 2 O] + , or other ions are used for quantification, the available amount of water within the ion source significantly affects probable ionization routes.Hence, n-alkanes cannot be characterized in a reliable way when the water concentration changes at the same time.Second, without proper cleaning procedures, hydrocarbon accumulation in the ion source may be severe.As long as no water is added, this accumulation remains rather unnoticed.However, additional water remobilizes these species in terms of C n H 2n+1 O + ions, which overlap with signals of the current sample.This leads to a significant interpretation problem when [M−3H+H 2 O] + ions are desired for n-alkane analysis, and the humidity level changes over time.At the same time, this means that water might be a suitable cleaning agent for accumulated hydrocarbons.
For reaction processes, where in addition to hydrocarbons, water is formed as a product (e.g., Fischer-Tropsch synthesis), we therefore suggest that the ion source be constantly enriched with water.This way, hydrocarbon accumulation should be reduced, and moreover, predominant ionization mechanisms are kept constant, i.e., no drastic signal changes can be expected when additional water is present within the product sample.An application to Fischer-Tropsch synthesis in currently in progress.A first proof of principle is given in the next section.

Measuring alkane/water mixtures under saturated humidity conditions
We performed another online monitoring experiment of n-decane and constantly moistened the argon carrier gas stream, this time with conventional H 2 16 O.The MS nitrogen supply was reset to the standard conditions of our previous study (sheath gas: 2; sweep gas: 10) [16].Similarly, in addition to N 2 , the same diluted syngas mixture (CO/H 2 / N 2 = 1:2:1, 200 ml STP /min) was applied as gas-phase matrix.Again, n-decane was injected with 1 µl/min, evaporated, and transported to the MS and GC instrument.A second syringe pump was installed to the evaporation zone of the setup, adding water (H 2 16 O) to the gas stream.Different water volume flows (1; 10; 30 µl/min) were applied to check the effect of varying water concentrations.
The temporal behavior of peak areas of C n H 2n+1 16 O + and C n H 2n+1 + ions is presented in Fig. 7.In addition, measured GC peak areas of n-decane (squares) are shown for qualitative reference.The water injection periods are marked by different background colors and volume flow annotations.In contrast to the previous experiments, no sharp change in signal levels was observed (either for C n H 2n+1 16 O + or for C n H 2n+1 + ), even during the highest water volume flow.Evidently, as envisaged, the ionization conditions seemed to be stabilized by the constant water enrichment of the argon carrier gas, so that additional water did not play a significant role.Hence, using a moistened carrier gas flow and thus keeping the ionization chamber at a high humidity level could allow for the analysis not only of n-alkanes but also of n-alkane/ water mixtures.However, data quality was quite poor in this experiment, as MS peak areas occasionally fluctuated quite heavily.When water was added to the gas stream, the GC data showed some outliers as well.This is probably due to an unsteady evaporation of C 10 and water, followed by an unequal flow split to the two instruments.Moreover, when comparing MS and GC data development, concentration changes of n-decane seem to be better captured by the alkyl fragment signals.The [M−3H+H 2 16 O] + (C n H 2n+1 16 O + with n = 10, m/z 157.1587) signal shows only a slight decrease during water addition and a less dynamic behavior.Hence, although accumulation is less pronounced, there still seems to be a memory effect during [M−3H+H 2 O] + measurement, which averages some dynamics.As the MS method aims for a rapid species analysis to measure system dynamics that are invisible in slow GC separations, this issue has to be further evaluated.Moreover, the suitability of a more complex sample composition (various hydrocarbons and water) as well as a changing syngas matrix still needs to be evaluated.Currently, a method application to online monitoring of a real Fischer-Tropsch product flow is in preparation.

Conclusion
In this work, we investigated the role of water during APCI(+) online mass spectrometry of n-alkanes.By switching between a dry and a wetted argon carrier gas for sample injection, as well as by using 18 O-labeled water, we revealed that water possesses multiple roles in the hydrocarbon ion formation.First of all, high humidity suppressed the ionization of alkyl (C n H 2n+1 + ) and alkenyl (C n H 2n−1 + ) ions.This indicates that proton transfer by H 3 O + was insignificant for C n H 2n+1 + and C n H 2n−1 + formation for volatile n-alkanes under the given ionization conditions.
Further, a dual role in the formation of oxygen-containing C n H 2n+1 O + ions was determined.Analyte-specific [M−3H+H 2 O] + formation was presumably based on reactions of primary analyte ions with adsorbed water molecules.During this process, it is assumed that molecules transfer a part of their internal energy to the walls of the ion source.This way, desorbing products did not suffer from major chain cleavages.However, this advantage of [M−3H+H 2 O] + ions was diminished by the fact that gaseous water reacted with adsorbed hydrocarbons (analytes and fragments) from previous sample injections to C n H 2n+1 O + ions, which overlap with signals from the current sample.Evidently, the continuous online sampling led to a severe analyte accumulation in the ion source.This remained rather unnoticed unless water was added.
For a method applied to online analysis of n-alkane mixtures, this accumulation or memory effect would cause significant interpretation problems, especially when the water concentration changes over time.A first solution approach, which stabilizes the ionization conditions and minimizes accumulation by keeping the ion source at a continuously high humidity level, was proven valid.This strategy could be promising for analyzing gas flows consisting of n-alkanes and water, as for instance in Fischer-Tropsch synthesis.Nevertheless, the reliability of this method in such a technically relevant application still needs to be demonstrated.

Fig. 1
Fig. 1 MS sample injection configuration.a Argon carrier gas usage with either (i) dry argon conditions or (ii) water-enriched argon.Additionally, the sample valve positions for (1) sampling and (2) injection are shown.b A typical plot of signal intensity over measurement time during one sample injection and the corresponding valve positions (1) and (2)

Fig. 2
Fig. 2 Exemplary mass spectra of a C 5 /C 6 /C 7 /C 10 /C 12 /C 14 sample injection using a dry argon as carrier gas and (b) H 2 18 O-enriched argon carrier gas 2 18 O injection."Second, analyte-specific [M−3H+H 2 O] + /[M−H+O] + ions were obviously not formed by reactions with gaseous water.If this were the case, C n H 2n+1 18 O + should have been detected for all analytes.Nevertheless, an increase in the peak areas of C n H 2n+1 16 O + was observed.Hence, the addition of gaseous water seems to enhance [M−3H+H 2 16 O] + formation via an indirect reaction pathway, i.e., by supporting an analyte

Fig. 4
Fig. 4 Temporal development of C n H 2n+1 18 O + signals during and after H 2 18 O injection

Fig. 5
Fig. 5 Peak areas of the ion groups a C n H 2n+1 16 O + , b C n H 2n+1 18 O + during a constant n-decane injection and a switch between dry (white background) and H 2 18 O-wetted (shaded back-

Fig. 6
Fig. 6 Proposed ionization scheme during APCI-MS analysis of volatile n-alkanes.The dual role of water is highlighted in red: adsorbed water (H 2 O(ads)) leads to the formation of [M−3H+H 2 O] + ions, which hardly suffer from further fragmentation (blue pathway).Gaseous water reacts in the form of H 3 O + (g) with adsorbed neutral hydrocarbons (former analyte or fragment molecules)

Fig. 7
Fig. 7 MS peak areas of a C n H 2n+1 16 O + and b C n H 2n+1 + ions (left scale) as well as GC peak areas (right scale) during an n-decane/water injection experiment.While C 10 was held at a constant volume flow show the temporal development of peak areas for the ion groups C n H 2n+1